I might see the answer to my own question here. This effect, far from negligible, produces a variation in gravity, resulting in gravity not being directed towards the centre of the Earth. But, since this is constant at a given latitute and varies only slowly across the surface it has no practical impact on motion in a given location. It's simply that gravity appears to act not quite towards the centre of the earth. But, since it's the same for everyone (it depends only on the angle of latitute, Earth's radius and angular momentum), it can be ignored.

Unlike the Coriolis effect, which will affect a pendulum over a long period of time or a falling object when it speeds up, the effect on the direction of gravity is essentially constant. So, Coriolis affects motion to a varying extent, so is significant in that respect.